1. Field of the Invention
The present invention relates to Rxe2x80x94Fexe2x80x94B base permanent magnet materials.
2. Prior Art
Rare-earth permanent magnets are commonly used in electrical and electronic equipment on account of their excellent magnetic properties and low cost. Lately, a need has been felt for the development of higher-performance magnets of this type. One family of rare-earth permanent magnets, namely, rare earth-iron-boron (Rxe2x80x94Fexe2x80x94B) magnets, has lower starting material costs than rare earth-cobalt (Rxe2x80x94Co) magnets because the key element neodymium exists in more plenty than samarium and the content of cobalt is low. This family of magnets also has much better magnetic properties than rare-earth cobalt magnets, making them excellent as permanent magnet materials.
Not surprisingly, there exists a desire for further increases in the magnetic properties of such Rxe2x80x94Fexe2x80x94B permanent magnets. Increasing the magnetic properties of Rxe2x80x94Fexe2x80x94B permanent magnets will require a reduction in the oxygen concentration within the constituent alloy. However, lowering the oxygen concentration within the alloy tends to result in abnormal grain growth during the sintering process, giving a magnet having a high residual flux density (Br), but an inadequate maximum energy product ((BH)max), and thus a hysteresis curve with a poor squareness.
It is therefore an object of the invention to provide high-performance Rxe2x80x94Fexe2x80x94B base permanent magnet materials that are comprised of alloys having lowered oxygen concentrations yet exhibit little abnormal grain growth.
We have succeeded in holding down grain growth within neodymium-based magnetic alloys produced for the most part in low-oxygen processes by uniformly dispersing and precipitating a finely divided zirconium compound, niobium compound or hafnium compound in a cobalt, aluminum and copper-containing Rxe2x80x94Fexe2x80x94B permanent magnet which contains also zirconium, niobium or hafnium, so as to obtain sintered permanent magnet materials with greatly improved magnetic properties and a much broader sintering temperature range.
More specifically, lowering the oxygen concentration within the alloy for a Rxe2x80x94Fexe2x80x94B permanent magnet tends to give rise to abnormal grain growth and restrict the range in the optimal sintering temperature, drastically reducing productivity. To overcome such problems, we tried adding trace amounts of new elements to the alloy.
As we described earlier in Japanese Patent Application Kokai No. 2000-234151, following extensive studies on the addition of new elements to copper-containing Rxe2x80x94Fexe2x80x94B permanent magnets and on the amounts of such addition, we found that the addition of a trace amount of zirconium can increase somewhat the residual magnetic flux density (Br) and can greatly increase the coercivity (iHc). Subsequently, upon endeavoring to lower the oxygen concentration in the production processes so as to further enhance the magnetic properties, we have discovered that the optimal sintering temperature range can be greatly expanded by inducing the fine and uniform precipitation of a zirconium-boron compound, a niobium-boron compound or a hafnium-boron compound within the magnet.
Accordingly, the present invention provides a Rxe2x80x94Fexe2x80x94B base permanent magnet material composed of a rare earth-iron-boron magnetic alloy which contains a Fe14R2B1 primary phase in a volumetric proportion of 87.5 to 97.5% and a rare earth oxide or a rare earth and transition metal oxide in a volumetric proportion of 0.1 to 3%; wherein the alloy has a metal microstructure containing as a major component a compound selected from the group consisting of zirconium-boron compounds, niobium-boron compounds and hafnium-boron compounds, which compound has an average grain size of at most 5 xcexcm and is uniformly distributed within the alloy such that the maximum interval between neighboring grains of the compound is at most 50 xcexcm.